I. Field of the Invention
This invention relates to methods and apparatus for belling plastic pipe used as conduits for fluids and electrical wiring.
II. Description of the Prior Art
Ever since the development of processes for extruding rigid thermo-plastic piping which is cut into lengths suitable for handling, shipping to an installation site where it is joined end to end with other pipelengths for use as a conduit for fluids and cables or other types of wiring, it has been the practice in the art to bell at least one end of a pipe section to provide such an increase in diameter for a short length of the end of the pipe section as to enable it to receive the unbelled end of another pipe section in a close fit. To accomplish such belling, a number of different types of apparatus operating in accordance with different methods have been devised, as exemplified by the teachings of the following patents:
______________________________________ NAME NUMBER ______________________________________ Niessner 3,205,535 Kuhlemann 3,557,278 Dalik 3,672,804 Osterhagen 3,806,301 Barnett 3,861,847 Ronden 3,910,744 Emery 3,923,443 Emery 3,932,094 ______________________________________
While the apparatus and methods taught by each of the foregoing patents, as well as in other patents, or otherwise sold or used in the industry may be satisfactory for accomplishing their particular objective or objectives, there has always existed with plastic pipes, no matter by what method or apparatus of the prior art it has been belled, an annular interruption or gap where the end of the inserted pipe end seats inside a belled pipe end. In many applications the existence of such interruption or gap poses no real problem, as, for example, where the inside diameter of the pipe may be quite large and it is intended to carry a relatively slow-moving fluid and some fluid leakage may be tolerated. However, in situations where the pipe walls may be thick in order to accommodate fluid under high pressure, where the inside diameter of the pipe may be relatively small and the velocity of the fluid passing through the pipe may be high, where no leakage may be tolerated through the pipe joint, or where wires may be pushed through the joined pipes to be pulled out at the other end, the existence of such interruptions or annular gaps at the annulars where the inserted pipe end abuts the widened wall of the belled pipe end, may present special problems to the installer and/or user of said plastic piping. Moreover, recently published specifications of the Underwriters Laboratories for belled plastic pipe intended to carry electrical cables for wiring, have specified such close tolerances with respect to both the belling angle as well as the axial width of the annular gap that most plastic piping belled by prior art methods and apparatus may be found to be incapable of meeting such Underwriters Laboratory specifications.
Another problem with prior art belling methods is that whenever a cylindrical pipe end, after heating, is forced outwardly to assume the belled configuration, necessarily the distension of the cylindrical walls results in their thinning where the radial expansion takes place. While in some situations the thinning of the pipe walls in the belled area may not be objectionable, there are other applications for belled pipe, as for example, where the piping is intended to carry fluid under extremely high pressure, that any thinning of the pipe walls may become quite unacceptable. In anticipation of such wall thinning through the belling process, some piping extruders have provided specially thickened annular areas in the piping as it is extruded. However, where this is done, it becomes necessary for the pipe to be cut only at such thickened areas. Thereby, the lengths of the pipe which may be produced are limited to the settings of the extruder as it produces such thickened areas in the piping emanating from it.
There has thus resulted a considerable need in the plastic piping industry to devise a method and apparatus for belling plastic pipe to more defined configurations and to much closer tolerances than have been heretofore obtainable by prior art methods and apparatus.
There has also existed a need for a method and apparatus to bell piping in such a manner that the resulting walls of the belled end of the pipe are not objectionably thinned, as has heretofore been the case, so that, as hereinbefore explained, it has been necessary to compensate for such thinning by having the extruder produce annular thickened areas of the pipe.